RequestBody.create 接收封装的json数据
 MediaType.parse(“application/json”) 上传json文件的类型
findViewById(R.id.bt1).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                OkHttpClient okHttpClient = new OkHttpClient();

                //MediaType.parse("application/json")封装上传文件的类型和json数据

                RequestBody requestBody = RequestBody.create(MediaType.parse("application/json"), "{\"a\":1,\":2}");

                Request request =new Request.Builder().url("https://www.httpbin.org/post").post(requestBody).build();

                Call call=okHttpClient.newCall(request);

                call.enqueue(new Callback() {
                    @Override
                    public void onFailure(Call call, IOException e) {

                    }

                    @Override
                    public void onResponse(Call call, Response response) throws IOException {
                        if (response.isSuccessful()) {//判断是否返回成功
                            Log.i("TAG", "Get异步请求" + response.body().string());
                        }
                    }
                });
            }
        });